Treatment of disease by prolonged delivery of an active agent at a controlled rate has been a goal in the drug delivery field. Various approaches have been taken toward delivering the active agents.
One approach involves the use of implantable diffusional systems. For example, subdermal implants for contraception are described by Philip D. Darney in Current Opinion in Obstetrics and Gynecology 1991, 3:470-476. Norplant.RTM. requires the placement of 6 levonorgestrel-filled silastic capsules under the skin. Protection from conception for up to 5 years is achieved. The implants operate by simple diffusion, that is, the active agent diffuses through the polymeric material at a rate that is controlled by the characteristics of the active agent formulation and the polymeric material. Darney further describes biodegradable implants, namely Capranor.TM. and norethindrone pellets. These systems are designed to deliver contraceptives for about one year and then dissolve. The Capranorm.TM. systems consist of poly(.epsilon.-caprolactone) capsules that are filled with levonorgestrel and the pellets are 10% pure cholesterol with 90% norethindrone.
Implantable infusion pumps have also been described for delivering drugs by intravenous, intra-arterial, intrathecal, intraperitoneal, intraspinal and epidural pathways. The pumps are usually surgically inserted into a subcutaneous pocket of tissue in the lower abdomen. Systems for pain management, chemotherapy and insulin delivery are described in the BBI Newsletter, Vol. 17, No. 12, pages 209-211, December 1994. These systems provide for more accurately controlled delivery than simple diffusional systems.
One particularly promising approach involves osmotically driven devices such as those described in U.S. Pat. Nos. 3,987,790, 4,865,845, 5,057,318, 5,059,423, 5,112,614, 5,137,727, 5,234,692 and 5,234,693 which are incorporated by reference herein. These devices can be implanted into an animal to release the active agent in a controlled manner for a predetermined administration period. In general, these devices operate by imbibing fluid from the outside environment and releasing corresponding amounts of the active agent.
The above-described devices have been useful for delivering active agents to a fluid environment of use. Although these devices have found application for human and veterinary purposes, there remains a need for devices that are capable of delivering active agents, particularly potent unstable agents, reliably to a human being at a controlled rate over a prolonged period of time.